1. Field of the Invention
The present invention relates to a method of manufacturing an epitaxial wafer, more specifically, a method of manufacturing an epitaxial wafer in which a circular susceptor is used and an epitaxial film is grown on a front surface of the wafer in a vapor-phase epitaxial method.
2. Description of Related Art
With increasing wafer diameter these days, a single-wafer type vapor-phase epitaxial growth apparatus is widely used, in order to grow an epitaxial film on a front surface of a silicon wafer. In a single-wafer type apparatus, a silicon wafer is first placed on a susceptor installed in a passageway-shaped reactor (chamber). Subsequently, when being heated by a heater provided external to the reactor, the silicon wafer is reacted with a variety of source gases (raw material gas and reactive gas), which pass through the reactor. Thereby, an epitaxial film is grown on a wafer front surface. A widely used susceptor has a circular shape from a plan view, on which a single wafer is mountable. The type of susceptor is used in order to evenly heat a wafer having a large diameter, such as, for example, a circular silicon wafer having a diameter of 300 mm, and to supply source gas on an entire wafer front surface; and thereby to evenly grow an epitaxial film. A wafer housing portion having a recess shape is provided in a middle portion of an upper surface of the susceptor, so as to house a silicon wafer having front and rear surfaces positioned horizontally. A recent susceptor generally supports a silicon wafer in a boundary area with a chamfered surface of a rear surface of the silicon wafer (for example, Related Art 1). In order to provide a wafer supporting position in the boundary area, one method is to evenly reduce a thickness of a middle portion of a bottom plate of the wafer housing portion, and thereby to provide a step around an external peripheral portion of the bottom plate. The other method is to cut out in a circular shape the middle portion of the bottom plate of the wafer housing portion, and thereby to provide the bottom plate having a ring shape. The boundary area means an area of less than 1 mm internally and externally in a direction of a wafer radius, centering a boundary line with the chamfered surface of the rear surface of the silicon wafer.
Silicon carbide (SiC) has conventionally been employed as material of a susceptor front surface. Thus, the susceptor has a greater hardness than the silicon wafer (Vickers hardness: SiC=2,200 to 2,500 HV; Si=1,050 HV). Further, the susceptor has a higher coefficient of thermal expansion than the silicon wafer, as the coefficient of thermal expansion of SiC is 4.8×10−6/k and that of silicon is 2.5×10−6/k. The boundary area of the wafer rear surface and an upper edge of an internal periphery of the external peripheral portion of the bottom plate of the wafer housing portion are in friction at a time of epitaxial growth, when a temperature inside the chamber is high. Due to the difference in the coefficient of thermal expansion, scratches are caused at the time in the boundary area of the silicon wafer, which is softer than the susceptor. The scratch has a groove-like shape similar to a hangnail caused in a portion that rims a base of a nail (hangnail injury). A planar shape of the scratch is a line, a dot, and the like. A cross-sectional shape thereof is a V-shaped notch and the like.
Related Art 1: Japanese Patent Laid-open Publication No. 2003-229370
With microprocessing in a device process, however, problems described below occur when scratches exist on the rear surface of the silicon wafer in the boundary area between the chamfered surface and the wafer rear surface. Specifically, when the silicon wafer is immersed in an etching solution and the like in the device process, particles are generated from the scratched portion. The particles then move to a front surface (device formed surface) side of the wafer, and are deposited on the wafer front surface. Thus, a yield in the device process is reduced.